1. Technical Field
The present invention relates to an improvement of a deconvolution processing method for eliminating a blur of a reconstructed image in an X-ray tomographic apparatus.
2. Background Art
The schematic construction of an X-ray emission mechanism and an X-ray detection mechanism of an X-ray tomographic apparatus is illustrated in FIG. 3. That is, the X-rays emitted from an X-ray tube are shaped into a fan beam by a collimator 2, transmitted through a reconstruction region 3 in which is placed a body or an object and impinge on an X-ray detector 4 so that the strength of the X-rays incident on respective channels is detected as an electrical signal. As shown in FIG. 4, the electrons 31 emitted from a cathode (not shown) impinge on a target 7 on a rotating anode 6 so that the bombardment energy of the electrons is converted into the X-ray beam 32 which is radiated. Reference numeral 8 designates a focal point on the target 7 which radiate the X-rays in response to the bombardment by the electrons; 9 designates an anode shaft supporting the target 7 which is rotatably supported by bearings 10. The focal point 8 has an extremely small area so that the X-ray radiation becomes the plane radiation. Of the electrons radiated toward the focal point 8, some electrons are rebounded from the focal point 8 and impinge again on the surface of the target 7 except the focal point so that the X-rays radiated from afocal points other than the focal point 8 are produced. The X-rays radiated from the plane focal point and the X-rays radiated from the afocal points are the cause for bluring an image obtained by the tomographic apparatus.
In order to eliminate such blur, the data detected from each channel of the X-ray detector is subjected to the deconvolution processing which will be described briefly below. For instance, when the X-rays are radiated by locating a pin target in the reconstruction region 3 in the mechanism shown in FIG. 3, the X-ray detection signal corresponding to the shadow of the pin target is obtained from the X-ray detector 4 as shown in FIG. 5. In FIG. 5, the abscissa designates the positions of the channels of the X-ray detector while the ordinate represents the X-ray strength. Reference numerals 21 and 22 denote the shadows of the pin and 21 indicates the umbra while 22, the penumbra which causes a blur. The function used to express the profiles such shadows is PSF (Pin Spread Function) which represents the characteristic of the X-ray tube. In order to eliminate a blur, the data of all the channels of the X-ray detector are subjected to the deconvolution processing by utilizing the inverse function obtained in correspondence with PSF. That is, in correspondence with the PSF obtained as a function of the positions of the channels as shown in FIG. 6A, an inverse function 23 which is a function of the positions of the channels is obtained as shown in FIG. 6B. When the deconvolution processing is carried out by using this inverse function and the data of the channels in a range in which is defined a function, the channel data 21 excluding any bluar can be obtained as shown in FIG. 6C.
When the convolution processing is carried out in the manner described above, the skirt of the PSF spreads over many channels because the prenumbral portion due to the radiation of the X-rays from the afocal points extends over a wide range. As a result, the skirt of the inverse function also spreads accordingly so that the channel data which are used in one convolution operation are increased in number. Furthermore, the number of channels of the recently developed tomographic apparatuses is increased and since all the channel data must be subjected to the convolution operation, the number of data to be processed is increased. When the number of channel data used in one convolution processing is increased so that the number of data to be processed is increased, the load on the computer for carrying out the convolution becomes heavy, thereby hindering the image reconstruction at a high speed.